Estimates of nitrogen input to chronically nitrogen-deficient sand-dunes were made at Woodhill Forest, near Auckland, using a site recently replanted with Pinus radiata
D. Don. The study was aimed at screening three legumes—Maku lotus (Lotuspedunculatus
Cav. "Grasslands Maku"), hairy canary clover (Dorycnium hirsutum
(L.) Ser.), and everlasting pea (Lathyrus latifolius
L.)—as possible nitrogen-fixing replacements for yellow tree lupin (Lupinus arboreus
Sims). Seasonal biological nitrogen fixation (BNF) of the legumes was compared using the isotope dilution technique and above-ground dry matter productivity. Sampling was conducted in winter (July) and spring (November) of 1994, and summer (February) and winter (July) of 1995.
Dorycnium hirsutum plots contained the highest amount of legume dry matter and this did not differ significantly between seasons. When woody components were omitted, La. latifolius was the most productive species. Dorycnium hirsutum and La. latifolius derived, on average, 98% and 95% respectively of their annual nitrogen uptake from the atmosphere (%Ndfa). This was not significantly affected by season. Lotus pedunculatus, on the other hand, showed a lower %Ndfa in summer.
Lathyrus latifolius was found to contain more fixed nitrogen (214 kg/ha/year) above ground than Lo. pedunculatus or D. hirsutum (55 and 71 kg/ha/year respectively). In all legumes studied, high rates of nitrogen fixation were observed between winter and spring and BNF was highly correlated with dry matter production.
Overall, the results showed that La. latifolius may be considered as a replacement for lupin in the ecosystem studied because of its persistence, its capacity for high non-woody dry matter production, its dependence on nitrogen derived from the atmosphere, and its high nitrogen fixation rate.